The purpose of this study is to examine cortisol as a mediator of psychosocial prenatal stress and early infant temperament in humans. The most extensive examination of this issue has been conducted in animals. Research in animals has demonstrated that offspring exposed to prenatal stress went on to exhibit fearful behaviors, learning difficulties, and problems with physical and motor development during infancy and into adulthood (Meek, Burda, & Paster, 2000; Schneider et al., 1992, Grimm & Frieder, 1987). Further work in this area has also demonstrated that the effects of prenatal stress on offspring development are mediated by the physiological stress response of the maternal hypothalamic-pituitary-adrenal (HPA) axis (Barbazanges, Piazza, le Moal, & Macccari, 1996; Weinstock, 1997). Although evidence suggests a similar relationship in humans, this relationship has yet to be explored. Therefore, using a prospective research design with 150 participants, this study will examine the stres hormone cortisol as a mediator of psychosocial prenatal stress and infant temperament. Specifically, salivary cortisol and self report measures of stress and infant temperament will be used to evaluate this relationship.